ASU researcher may have answer for why cave animals go blind

Study undermines Darwin's suggestion of 'disuse'

April 17, 2017

Why do animals that live in caves become blind? This question has long intrigued scientists and been the subject of hot debate.

Clearly, across the animal kingdom, blindness has evolved repeatedly.  There are thousands of underground and cave-dwelling species, from naked mole rats to bats, and many have lost their sense of sight.  A well-studied blind cavefish (bottom), the Mexican tetra (Astyanax mexicanus), is a small, docile, pink-hued fish just a few centimeters long that could easily make its home in an aquarium. ASU evolutionary biologist Reed Cartwright chose this Mexican tetra because there is also a surface-dwelling form (top) that has retained its sight. Photo courtesy of Martina Bradic, New York University Download Full Image

Charles Darwin originally suggested that eyes could be lost by “disuse” over time. But Reed Cartwright, an ASU evolutionary biologist in the School of Life Sciences and researcher at the Biodesign Institute, may be proving Darwin wrong in a recent publication in the journal BMC Evolutionary Biology.

“We think that blindness in cavefish is indeed Darwinian, but ultimately this disproves Darwin’s original hypothesis of ‘disuse’,” Cartwright said. In new research, he explains that eyes are not lost by disuse, but rather a demonstration of Darwin’s fundamental theory of natural selection at work — with blindness selected as favorable and the fittest — for living in a cave.

Go fish

For their work, his research team choose to model a well-studied blind cavefish, the Mexican tetra (Astyanax mexicanus), a small, docile, pink-hued fish just a few centimeters long that could easily make its home in an aquarium.

It’s inhabited caves for 2 million to 3 million years, giving it 5 million generations worth of time to evolve blindness. Cartwright’s group chose this Mexican tetra because there is also a surface-dwelling form that has retained its sight. And for scientists, this built-in comparative power makes it a good choice for further exploration. They have two populations to study that can interbreed and are polar opposites for physical traits.

So Cartwright’s group decided to use computational power to investigate how multiple evolutionary mechanisms interact to shape the fish that live in caves.

“The problem we have in these caves is that they are connected to the surface, and fish that can see immigrate into the cave and bring genes for sight with them,” Cartwright said. “Under these conditions, we don't typically expect to find such a difference in traits between surface and cave populations. Unless selection was really, really strong.”

How strong? In their model, the selection for blindness would need to be about 48 times stronger than the immigration rate for Mexican tetras to evolve blindness in caves. Cartwright’s group estimates that a measure of fitness for blindness, called the selection coefficient, in the tetra is between 0.5 percent and 50 percent.  

These coefficients are high enough that laboratory experiments should have detected a difference between surface and cave forms of the fish; however, none have to date.

Why do animals become blind? Charles Darwin originally suggested that eyes could be lost by disuse over time. But Reed Cartwright, an ASU evolutionary biologist in the School of Life Sciences and researcher at the Biodesign Institute may be proving Darwin wrong in a recent publication in the journal BMC Evolutionary Biology.

Blinded by the light

Cartwright’s team turned to a hypothesis going all the way back to a letter to the editor of Nature in 1925 by E. Ray Lankester, that essentially stated that the reason you have blindness in caves is because the fish that can see simply leave.

“If sighted fish swim towards the light, the only fish that stay in the cave are blind fish. They aren’t trying to get to the light anymore because they can’t see it. Which actually is a form of selection, and thus, Darwinian evolution in action,” Cartwright said.

According to Cartwright, explaining a fitness difference as big as 10 percent between sighted and blind fish may be difficult, “Iosing eyes might not give you 10 percent more offspring. However, if 10 percent of your seeing-eye fish leave the cave, the migration rate is reasonably low, and that could be enough.”

If over time, enough of the seeing-eye fish are systematically being removed, they will also be removed from the gene pool, and that could be enough to drive the evolutionary process.

It could be this sort of habitat preference that maintains the local blind fish population, and the fish that can see are preferentially moving out of the cave. “We found that even a low level of preferential emigration, e.g. 2 percent, would provide a significant boost to local adaptation and the evolution of blindness in caves.”

Cartwright’s team hopes that field biologists begin to consider Lankester's 90-year old hypothesis when studying cavefish. “It would be great if someone could develop a study to test Lankester's hypothesis and whether it is driving the evolution of blindness in caves. That would really help answer one of the questions that have intrigued biologists for over a century.”

Cartwright’s research was supported by National Science Foundation Advances in Bioinformatics program and Arizona State University’s School of Life Sciences and Barrett, The Honors College

Joe Caspermeyer

Manager (natural sciences), Media Relations & Strategic Communications


ASU dance students to perform new works by renowned choreographers at SpringDanceFest

April 17, 2017

This year, ASU's School of Film, Dance and Theatre’s annual SpringDanceFest concert will not only feature the choreography of dance students, but also the choreography of three recent guest artists, who created new works for students while visiting Arizona State University.

“This has been an incredible year for us with the guest artist program,” said Mary Fitzgerald, assistant director of dance in the School of Film, Dance and Theatre in the Herberger Institute for Design and the Arts. “Three renowned choreographers with quite distinct aesthetics based in urban and postmodern dance forms created new works for a total of 35 students. Each artist was on campus for two-to-three weeklong residencies and taught a range of classes in the dance and theatre areas for more than 100 students.” ASU's SpringDanceFest poster Download Full Image

These artists include MacArthur Award recipient Kyle Abraham, urban dance artist Teena Marie Custer and postmodern/experimental choreographer Jesse Zaritt. Each choreographer created large ensemble pieces that use a range of aesthetics to explore spirituality and politically charged issues about race, identity and oppression.

“Their works represent some of the most cutting-edge dance-making in the field,” Fitzgerald said. “I am really impressed by the range of voices represented and by the very different ways that these choreographers challenged our students artistically."

SpringDanceFest also features choreography from several students, who continue that range of representation and who found inspiration in various places, from feminist literature and dance films to the desert and “Romeo and Juliet.”

First-year graduate student Laina Carney’s piece “Untitled: Part II” is the second part of her Untitled Series.  

“This series was inspired by the book ‘Bad Feminist’ by Roxane Gay as well as by the contextualization of popular culture’s perspective of the ‘modern’ female through time,” Carney said. “The piece for SpringDanceFest features five dancers and uses blended aesthetics of contemporary modern dance and pedestrian sub-cultures to challenge notions of identity, gender and body attitudes through movement.”

In the piece, lighting designer Lacee Garcia uses visual counterparts to take the audience through a timeline of socially constructed modes of female complexities, Carney said.

“I hope that the work will stick with the audience for longer than the duration of the piece, and also, that it allows them to reflect upon gender roles in today’s society in a new way.”

Michelle Marji, a senior studying dance and psychology, used her surroundings and her dancers to choreograph “Desert Dance.” 

“When creating this piece, I drew inspiration from the desert and the emotional experiences of my dancers,” Marji said. “We used meditation and imagery to create intention in the piece. Ultimately, the desert acts as a symbolic representation of our emotional experience – dancers are affected by the environment they live in (the desert), their emotional human existence, the sound and each other. They affect each other's experience and go through a tumultuous journey before quenching their thirst in the desert.”

Jordan Klitzke’s “This Is Only Temporarily New” is a contemporary look at the “Romeo and Juliet” balcony scene.

Danced by two undergraduate female students, the duet brings the experience of the body to the forefront and lets the text become secondary, allowing a “look at the power dynamics of young relationships and how those are primarily established through subtle and not-so-subtle actions of the body,” Klitzke said.

“I hope the audience gets lost in the world the dancers so beautifully established through their strong, intimate connection with the text, the movement and each other,” Klitzke said. “I'm not looking for the audience to understand something specific I'm trying to say – I am interested in presenting a world on stage where people appreciate the intense beauty before them and enjoy themselves in unexpected ways.”

Other pieces in the SpringDanceFest program include “Holding On” by Arielle Lemke, “Adieu” by Yingzi Liang, “Listen” by Alexus Purnell, “beauty: /ˈbyo͞odē/ noun 1.  a state of being” by Mac Allen and “The Time is Now” by Shelley Jackson in collaboration with dancers.

SpringDanceFest runs at 7:30 p.m. April 21–22 and 2 p.m. April 23 at the Paul V. Galvin Playhouse. For more information and to buy tickets, visit ASU Events

Sarah A. McCarty

Marketing and communications coordinator, Herberger Institute for Design and the Arts